Proc. International Conference on Space Optical Systems and Applications (ICSOS) 2014, S7-3, Kobe, Japan, May 7-9 (2014)
Characterization of 64 pixel NbTiN superconducting
nanowire single photon detectors
Shigehito Miki, Taro Yamashita, Hirotaka Terai
Zhen Wang
Advanced ICT Research Institute, Nano ICT group
National Institute of Information and Communications
Technology
588-2, Iwaoka, Nishi-ku, Kobe, Hyogo 651-2492, Japan
line 4-e-mail address if desired
Shanghai Center for SuperConductivity
Shanghai Institute of Microsystem and Information
Technology
R3317, 865 Changning Road, Shanghai 200050, PR China
Abstract— We present the characterization of twodimensionally arranged 64-pixel NbTiN superconducting
nanowire single-photon detector (SSPD) array. NbTiN films
deposited on thermally oxidized Si substrates enabled the highyield production of high-quality SSPD pixels, and all 64 SSPD
pixels showed uniform superconducting characteristics within the
small range of 7.19–7.23 K of superconducting transition
temperature and 15.8–17.8 µA of superconducting switching
current. Furthermore, all of the pixels showed single-photon
sensitivity, and 60 of the 64 pixels showed a pulse generation
probability after photon absorption higher than 90%.
Keywords— Single photon detectors, superconducting nanowire,
single photon detector array
I.
Introduction
Recent progress in improving the performance of
superconducting nanowire single-photon detectors (SNSPDs
or SSPDs [1]) are nothing short of eye-opening. At present,
their system detection efficiency (SDE) is reaching unity at a
wavelength of 1550 nm with a dark count rate (DCR) lower
than 1 kcps [2-4], and a short-system timing jitter of 20–40 ps
can be obtained [5]. These values are strikingly superior to
those obtained with commonly used avalanche photo diodes
(APDs) at telecommunication wavelengths [6]. In fact,
practical SSPD systems are already employed in a variety of
applications such as quantum key distribution, quantum optics
experiments, free space laser communication experiment, and
so on [7-13]. However, one remained issue is slow response
speed due to large kinetic inductance in superconducting
nanowire. To apply SSPDs into high speed free space laser
communications, the improvement of response speed is
essential. Dividing the sensitive area into the independent
multi pixels is an promising configuration to resolve this issue,
because the kinetic inductance of each pixel can be reduced by
the factor of N and the probability of photon incident also can
be reduced by the factor of N, where the N is the number of
pixels. As result, the maximum counting rate can be roughly
increased by the factor of N2.
One of the critical issues to realizing large-format SSPD
arrays is how to reduce heat flow from room temperature
through coaxial cables, the number of which increases as of
Copyright (C) ICSOS2014. All Rights Reserved.
the number of pixels increases in a conventional readout
scheme. Therefore, our primary effort so far has been focused
on the development of cryogenic readout electronics using a
single flux quantum (SFQ) circuit because the required
number of cables can be drastically reduced by implementing
SFQ circuits and SSPDs in a cryocooler system. We have
confirmed correct SFQ operation with output signals from
SSPDs [14], and we have succeeded in implementing a fourpixel SSPD array and SFQ circuit in a Gifford–McMahon
(GM) cryocooler system with no serious crosstalk [15].
Accordingly, scaling the number of SSPD pixels should be a
next step, which will require significant uniformity of
superconducting nanowire characteristics. Because our recent
development of single-pixel NbTiN SSPDs prepared on a
thermally oxidized Si substrate can provide high SDE devices
with high yield [4], it is natural to apply this technology to
scale up the SSPD arrays. In this work, we report the
development of two-dimensionally arranged 64-pixel NbTiN
SSPD array and characterization of electrical and optical
properties in a GM cryocooler system.
II.
Experimental Procedure
Figure 1 (a) shows the microphotograph and (b) shows the
the scanning electron micrograph (SEM) of the 64-pixel
NbTiN SSPD device. The NbTiN nanowire pixels were
fabricated on a Si substrate with a 250-nm-thick thermally
oxidized SiO2 layer. The fabrication process of the NbTiN
SSPDs was basically the same as described elsewhere [4,16].
The 5-nm-thick NbTiN nanowire was formed to be 100 nm
wide and 100 nm spaced meandering lines covering an area of
5 × 5 µm, thus configuring one nanowire pixel. The 8 × 8
nanowire pixels were two-dimensionally arranged with
spacing of 3.4 µm, covering an area of 63 × 63 µm. The 200nm-wide interconnection lines were formed using the same 5nm-thick NbTiN nanowires in the spaces between the
nanowire pixels, in which the width of interconnection lines
was two times wider than that of the nanowire pixel in order to
prevent a response to single-photon incidence. The
interconnection lines were then connected to coplanar
waveguide (CPW) lines. Since the dc resistance of long (~2
Proc. International Conference on Space Optical Systems and Applications (ICSOS) 2014, S7-3, Kobe, Japan, May 7-9 (2014)
The 64-pixel SSPD array chip was then mounted on the
chip-mounting block with a specifically designed printed
circuit board (PCB), and each nanowire pixel was wire bonded
to a 50 Ω microstrip line on the PCB, as shown in Fig.1 (c).
Since the total number of coaxial cables introduced into our
cryostat system was nine, we characterized the electrical and
optical properties of the 64 nanowire pixels by changing the
connections between the microstrip lines on the PCB and the
coaxial cables in turn. A single-mode optical fiber for a
wavelength of 1550 nm was introduced into the cryocooler
system, and the end of the fiber was fixed to the rear side of
the chip-mounting block after aligning the incident light from
the fiber with the device active area. The distance between the
fiber tip and the device active area (Lfiber-sspd) were set to 3 mm
to observe the incident photon response. The packaged block
was cooled with a 0.1 W GM cryocooler system, which can
cool the sample stage to 2.3 K [17].
(a)
63 µm
63 µm
(b)
5.0 µm
(c)
5.0 µm
To measure the SDE, a continuous tunable laser was used
as the input photon source. The wavelength of the light source
was fixed to 1550 nm and attenuated so that the photon flux at
the input connector of the cryostat was 109–1011 photons/s,
where the linearity of the output counts to the incident photon
flux were confirmed in advance. A fiber polarization
controller was inserted in front of the cryocooler’s optical
input in order to control the polarization properties of the
incident photons so as to maximize the SDE. The SDE was
determined by the relation SDE = (Routput – RDCR)/Rinput, where
Routput is the SSPD output pulse rate, RDCR the dark count rate,
and Rinput the input photon flux rate to the system. The SDE of
each pixel was measured individually by changing the
connection of the readout components at the outer side of the
cryocooler system.
III.
Results
Figure 2 shows the histograms of the observed Tc and Fig.
3 shows the superconducting switching current Isw, for the 64
nanowire pixels. As shown in the figures, both Tc and Isw of all
pixels fell within the small ranges of 7.19–7.23 K and 15.8–
17.8 µA, respectively. These results indicate that all pixels
were fabricated uniformly without significant defects in terms
of their electrical properties. We then characterized the optical
responses of all pixels. Figure 4 shows the histogram of
maximum pulse generation probability in each pixel (Ppulse,max),
where the pulse generation probability is defined as the
probability that the detector generate the output signal after
photon absorption into the nanowire. The Ppulse,max were
derived from the fitting from the bias dependencies of system
detection efficiency (SDE) using a sigmoid function [4,18].
The sigmoid shape for the bias current dependence of the SDE
has been shown empirically, and it is also well fitted to our
devices in this work [2,4,19]. As shown in the figure, 60 of the
64 pixels exceeded 90% of approximated Ppulse, indicating that
our 64 pixel arrays can operate with uniformly high sensitivity.
20
Number of Devices
mm) CPW lines could lead to a disturbance of the correct
operation of the SSPD, 100-nm-thick NbN films with a
superconducting transition temperature (Tc) of ~15 K were
used to assure zero dc resistance.
15
10
5
0
7.1
7.2
7.3
Tc (K)
Fig.1 (a) Microphotograph of 64-pixel NbTiN SSPD array. (b)
SEM image of one SSPD pixel. (c) photograph of 64-pixel
SSPD array chip installed in a mount block.
Copyright (C) ICSOS2014. All Rights Reserved.
Fig. 2. Histogram of superconducting critical temperature (Tc)
of 64 pixel SSPD array.
Proc. International Conference on Space Optical Systems and Applications (ICSOS) 2014, S7-3, Kobe, Japan, May 7-9 (2014)
Note that the system detection efficiencies of 64 pixel
arrays in this measurement were quite low (< 10-3%) due to
large losses before the absorption to the nanowire. For
example, the coupling efficiency between incident light and
sensitive area is low (~10-3%) because the Lfiber-sspd is long of
~3 mm and the 3.4 µm-wide spaces between nanowire pixels
for interconnections to CPW lines. In addition, optical cavity
structure did not attached on the nanowire, resulting in lower
absorptance as compared to our standard single pixel devices.
We believe if we can reduce these losses, the SDE of 64 pixels
can be increased drastically and will reach to the similar
values to that with standard single pixel devices.
Number of Devices
15
IV.
We characterized the electrical and optical properties of a
64-pixel NbTiN SSPD array prepared on a thermally oxidized
Si substrate. Our two-dimensionally arranged 64-pixel SSPD
array exhibited uniform superconductivity in all pixels and
pulse generation higher than 90% in 60 of the 64 pixels. Our
next study will be a simultaneous operation of all of 64
nanowire pixels with an SFQ signal processing circuit for high
speed operation. Of course, although further improvement of
bias current dependencies in the SDE such as those achieved
by WSi-SSPDs at a low operation temperature of ~300 mK is
more favorable for retaining the variations of Ppulse even at low
bias current regions [2], the results of this work provide
insights into realizing large-format SSPD arrays even at
operating temperatures of 2–3 K.
Acknowledgment
10
The authors thank Saburo Imamura and Makoto Soutome
of the National Institute of Communications Technology for
their technical support.
5
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Isw (µA)
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Fig. 3. Histogram of switching current (Isw) of 64 pixel SSPD
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Fig. 4. Histogram of maximum pulse generation probability
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